A method and system of linear road sampling providing road traffic flow measurements

ABSTRACT

A computer implemented method providing measurements of traffic flow conditions on selected roads, wherein selection of roads is according to a linear sampling of roads and traffic measurements is provided from raw data being qualified by road users participating in the measurement scheme.

TECHNICAL FIELD

The present invention relates to a method and system thereof providingqualified traffic measurements by linear road sampling, wherein trafficflow measurements are qualified by human interaction with the system.

BACKGROUND

Monitoring and analysis of measured traffic flow conditions on cityroads are indicators that may identify for example upcoming trafficcongestions, or congestions that are about to dissolve, but also toidentify roads with less traffic problems being possible roads roadtraffic can be directed to when traffic congestions occur.

There are many examples of methods and systems measuring traffic flowconditions in prior art. For example, deployment of sensors likepressure sensors buried in the road body, or optical means on the sideof roads may count numbers of cars passing per time unit at selectedpoints at different sections of the roads. This is then a measurement ofthe traffic flow rate (number of cars pr. time unit) at the specificlocations of the measurements. However, linking traffic flow conditionsand measured traffic flow rates can be difficult. For example, if thereis a queue that stands still the measured traffic flow rate is zero.This parameter is also zero when there are no actual cars passing thepoint. Therefore, interpreting traffic flow rate may provide amisleading assessment of traffic flow conditions.

When a high traffic flow rate is measured this may indicate that thetraffic passes a measurement point with high or normal speed. However,as known in prior art, such a traffic flow condition can quickly turninto a severe queue problem if just one car accidently hits the brakesfor a second or two. Such short disruptions of the traffic flow is knownfrom traffic queue theories to be able to create long lasting queueproblems when the traffic density is high. Therefore, interpreting thetraffic flow conditions may also be misleading when the traffic flowrate indicates high throughput of cars. This specific traffic conditionis a vulnerable traffic situation.

U.S. Pat. No. 5,530,441 disclose a method and an apparatus measuringtraffic flows inside a crossing, or near the crossing, with a number ofcameras. The method and apparatus are capable of extracting vehicleswith a high level of accuracy. Overlap of vehicles identified by thecameras can be avoided by setting the field of a camera to exclude arange from the inflow portion to the vicinity of centre of the crossingbut to include a range from the centre to the vicinity of the outflowportion of the crossing. Accordingly, accuracy of traffic flowmeasurements can be improved. However, traffic flow conditions are notonly related to what happens around a crossing. An incoming traffic laneto the crossing may be empty of cars because there is a wide heavyweight transport truck blocking traffic on the incoming lane far awayfrom the crossing, or there has been an accident etc.

U.S. Pat. No. 7,480,560 disclose a method and apparatus providingtraffic density and traffic flow information obtained from wirelessdevices. The wireless devices may be wireless telephones having GlobalPositioning System (GPS) capabilities. A server interacts with thewireless telephones compiling the traffic density and flow informationfrom acquired GPS position readings. The traffic density and flowinformation is then available for dissemination to requesting wirelesstelephones, or other requesting clients capable of contacting theserver. However, it is difficult to interpret measurement of carmovements relative to other car movements on a same limited road segmentif they travel with significantly different speed. The accuracy of anassessment of traffic flow conditions like this is based on anassumption that all road users providing GPS measurements are actingmore or less the same way in the traffic.

US 20130211706 A1 disclose systems and methods estimating the speed of amobile radio terminal traveling on at least one road segment. Thesystems and methods may further determining one or more routes through aroad network that a mobile radio terminal could have travelled based ona sequence of wireless measurements from the mobile radio terminal. Inaddition, the systems and methods may be collecting and transmittingradio communications network measurements from a mobile terminal for usein a system of estimating a traffic flow speed on road segments. Inexamples of embodiments, the process of accumulating and communicating aplurality of wireless measurements is to a traffic flow measurementserver such that reporting of wireless measurements is minimized whenthe mobile radio terminal is stationary.

Another problematic situation may arise if a server is tracking forexample a taxi. It is common that a taxi may stop in the middle of theroad to pick up passengers. Interpreting this situation as an upcomingqueue condition is possible since the traffic flow rate measured by thetaxi is zero, and a few cars may start to pile up behind the stoppedtaxi. This is also an example of how difficult it can be to measuretraffic density. If cars starts to pile up as discussed above, thetraffic density is very high over for example 50 meters behind the taxi,while the traffic density may be assessed as moderate if measured over100 meter instead. A further problem with traffic density measurementsis that cars can be lumped together in dispersed lumps on a road.Further, tracking a taxi that is allowed to travel in traffic lanesdedicated to public transport could indicate high speed, low car densityand no problems on the highway when indeed the other lanes of thehighway is stacked with cars.

The human ability to interpret and assess complex traffic situations isbest illustrated by the fact that most cities having traffic controlsystems uses deployed live video surveillance cameras being viewed andinterpreted by human operators in traffic control centres. The trafficcontrollers can regulate for example traffic light settings to improvetraffic flow conditions in respective areas based on their impression ofcurrent traffic patterns and experience of traffic behaviour in the citythey control. However, such systems have few means of guiding specificroad users experiencing difficult traffic situations. Such systemsprovides no feedback to specific road users being located at specificlocations.

Another aspect of prior art traffic measurement techniques is thatregardless of how measurements are done, due to often rapid changingtraffic conditions like for example an accident, traffic flowmeasurements have a limited scope of validity over time.

From a road user perspective, it can be of help to be informed abouttraffic problems ahead of the road positions a road user is located onat any time. However, what a road user needs to know is where there areopen roads, i.e. roads with acceptable transport capacity, and not somuch, where it is not. It is of course possible to identify open roadswhen it is known where there are traffic problems. However, due to theongoing changing traffic conditions in a road system the “open roads” assuch is also changing over time. In prior art it is known how to updatecomputer coded maps wherein roads with problems can be coloured red, and“open” roads are coloured green, while roads being in a transitionalstate is for example coloured yellow.

Even though it is known how to report traffic conditions to road users,it is still a question related to the quality of the measurements andassessments of road traffic conditions. If the quality of themeasurements are low, then the benefit of receiving traffic relatedinformation is non-existing. On the contrary, such information may bemisleading information.

There are examples of navigation tools that can receive reports ofaccidents for example, and which will automatically identify routesaround the accident. However, it is usually the same algorithm used inmany navigation tools resulting in a situation wherein too many cars aredirected to the same alternative route resulting in a queue condition onthis alternative road. When there is a traffic congestion, it is also amatter of definition of defining what is an open road or a road withacceptable transport capacity. If most of the traffic is standing still,a road with some moderate traffic flow is much more attractive to usethan those that are blocked. Therefore, traffic flow rates may berelative measures. When measuring traffic flow conditions, the validityof respective parameters that are measured depends often on surroundingconditions as well as the situation at a specific measurement point.Further, an assessment of surrounding conditions around a measurementpoint is actually a rather fluffy concept. It is more or less just animpression of how it is. It is not an accurate measurable parametervalue. Even though when using a collection of different measuredparameters analytically to provide an indication of surrounding trafficconditions, the traffic is also subject to random non-predictable eventslike accidents and the condition estimate will not be valid.

Therefore, it is a need of a method and system thereof providingimproved traffic flow measurements and assessments of trafficconditions.

OBJECT OF THE INVENTION

In particular, it may be seen as an object of the present invention toprovide a method and system that overcome the problems of prior art byqualifying traffic flow measurements with human observation capabilitieslocated on premises of respective traffic flow measurements.

It is a further object of the present invention to provide analternative to prior art.

SUMMARY

Thus, the above described object and several other objects are intendedto be obtained in a first aspect of the invention by a method and systemthereof, wherein a configured computer server executes steps of a methodactivating road users participating in a traffic flow measurementscheme, wherein road users are instructed to drive between a firstselected junction to a second selected junction according to anoptimized selection of junctions in a geographical area.

According to an aspect of the invention, a computer-implemented methodproviding measurements of traffic flow conditions on roads selected bylinear road sampling comprises a server configured to execute steps of:

-   -   specify a region covering a road system to be measured;    -   identify sample junctions that cover the region according to a        specific rule of coverage;    -   create a plurality of equivalent sectors around each of the        sample junctions that cover the full circle around the        respective sample junctions;    -   for each sample junction and each of its sectors: identify        closest junctions in a sector and identify shortest paths        from/to the sample junction to the closest junctions (k);    -   store all paths into a directed graph, wherein the nodes        represent the junctions;    -   update traffic flow measurements and measurements qualified by        road users in nodes of the graphs by tracking cars following        respective shortest paths identified above.

FIGURES

The method and system according to the present invention will now bedescribed in more detail with reference to the accompanying figures. Thefigures illustrates examples of embodiments of the present invention andis not to be construed as being limiting to other possible embodimentsfalling within the scope of the attached claim set. Further, respectiveexamples of embodiments may each be combined with any of the otherexamples of embodiments.

FIG. 1 illustrate an aspect of the present invention.

FIG. 2 illustrates another aspect of the present invention.

FIG. 3 illustrates an example of embodiment of the present invention.

FIG. 4 illustrates a further result of the embodiment illustrated inFIG. 3.

FIG. 5 illustrates an example of a directed graph.

DETAILED DESCRIPTION

According to an aspect of the present invention, uncertainty of trafficflow assessments may be mitigated if humans located at the premises ofspecific traffic measurement points are qualifying the measurements.

According to a further aspect of the present invention, multiple roadusers carrying mobile terminals like a mobile phone integrated withGlobal Positioning System (GPS) transceivers can provide GPSmeasurements used as raw data for traffic assessments or analysis.Qualifying for example GPS measurements sent to a server by adding atext message incorporating a qualifier of the measurements like “goodconditions”, “starting to be difficult” etc. will provide a betterinterpretation of the measurements. It is also possible that the serverreceiving the GPS based measurements submits questions to respectiveroad users asking road users to answer “yes” or “no” to respectivequestions related to the surrounding traffic environment. For exampleanswering “yes” or “no” to standardized questions like “Is the trafficdensity around your current position moderate?” etc.

According to an aspect of the present invention, qualifiers submitted byroad users may be coded with unique number identifications, for example“10” is the identification of the qualifier “good conditions”.

However, it is also a question of how many road users needs to be partof a scheme like this to get proper traffic condition assessments of alarge city at given time intervals. A further question is if it isactually necessary to measure every road in the city. If a main roadhave traffic congestions, it is obvious that many side roads guidingtraffic onto the main road also have traffic problems. In such asituation, an interesting question would be if there were roads withless traffic problems providing a route around such congestions. Ifhumans are involved in the measurement scheme a human may be capable offinding such an alternative road based on actual observations as well aslocal knowledge about roads in the area in question. When the server istracking the road users travelling on alternative roads, the roads areidentified by the server and can be used to guide traffic, or part ofthe traffic, onto these alternative roads to other road users comingfrom behind the road user that makes the measurements and qualifies themeasurements.

Therefore, a further aspect of the present invention is that a servermay request a road user to travel from a point A to a point B. Theserver makes GPS based tracking and measurements of the movement of theroad user, and when the road user is coming for example to the point B,the server requests a qualifier of respective traffic relatedmeasurements from the road user. This may include subjective assessmentsby the road user, or a subjective prognosis of future traffic conditionsetc.

According to a further aspect of the present invention, by selectingstarting points A and ending points B according to a specificoptimization algorithm according to the present invention, the number ofroad users participating in the measurements may be limited whenassessing traffic conditions of a complete city, even when suchassessments are performed on a regular basis.

There is also an important bias of traffic patterns in cities due to thefact that city planners design special main roads carrying the burden oftraffic in and out of subareas of the city, or for traffic passing thecity, or for transport vehicles etc. In addition, it is possible toclose or restrict traffic in parts of a city during daytime, or duringrush hours etc. Some cities have also introduced rush hour fees makingit expensive to use roads at certain times of a day. In a sense, thisprovides planned traffic patterns.

Therefore, it is possible to link the above referenced points A and B toreflect such planned traffic patterns. It is for example also possibleto have live feedback from traffic control centres controlling trafficlight settings to ease traffic congestions in certain areas of a city.

FIG. 1 illustrate an example of a route 10 between respective junctions11, 12, 13, etc. How a road user will travel between the junctions canof course be random. However, if the road user wants to travel betweenthe junctions as illustrated in FIG. 1, an optimized route 10 can beviewed as an optimized result achieved with what is known in prior artas a shortest path algorithm. If a road segment of the route 10 isblocked, an optimized route is not available. A road user may then findand use an alternative road 15 for example when driving from junction 11to junction 12. If the server is tracking the GPS positions of the roaduser, the alternative road is registered in the server. Then thisalternative route is actually part of the optimized route in the meaningof providing transport capacity to the road user. This is an example ofthe shortcomings of static maps. Planning routes based on static mapsdoes not take into account available roads.

There are some facts and intuitive understandings derivable from thesituation in FIG. 1 based on common knowledge of road systems.

-   -   A shortest path algorithm identifies a shortest geographical        route between junctions A and a junction B.    -   If junction B is located geographically close to junction A on a        shortest path, it is unlikely that there is a further junction        located in between junction A and junction B on this shortest        path.    -   Junctions are fixed objects, and there are many possible        different routes between junctions on a plurality of roads        interconnecting junctions.    -   Road users are capable of identifying alternative routes through        an area based on observations of traffic patterns and local        knowledge of an area.

Therefore, a server may instruct road users to be part of a schememeasuring road conditions on a plurality of roads in and out ofjunctions being located at fixed geographical points. As a result, it ispossible to guide road users on alternative roads (which is a variableresource) around junctions (which are fixed resources) with problems, orwhich are connected to roads with problems.

However, when finding an alternative road the number of fixed junctionsnecessary to pass may grow, hence the number of junction pairs leads toa quadratic growth in computational cost. It is a problem of connectingmany points to many points. Therefore, instructing road users to driverandomly between fixed junctions may result in high computational cost.A city may have thousands of roads and junctions.

It is possible to make the assumption that if a server instructs a roaduser to drive from a point or junction A to another point or junction B,wherein the geographical distance between A and B is short, the roaduser will probably follow a direct shortest road connecting A and B.Then there is a low probability that there is a further junction locatedin between A and B. Therefore, following short stretches of roads inbetween closely located junctions, results in measurements related onlyto conditions on roads between junctions. If a road between junction Aand B is intersected by another road the crossing of this road may havetraffic interfering with the measurements of the road between A and B.The traffic between A and B may be low, but if the traffic on thecrossing road is very high, the traffic in the junction between theroads may block the traffic on the road between A and B. This would thencorrupt the measurements on the road between A and B.

Based on this assumption, it is possible to select a plurality of pairsof junctions A and B wherein the distance between A and B is shortestpossible. It is further possible to make the assumption that there is noknown existing road between junction A and junction B since it ispossible that the existing road is blocked. Therefore, when instructinga road user to drive between a pair of junctions A and B, the road userdriving a car is using his navigational skills on the ground to follow aroute of his selection. Since the distance is short, it is a very highprobability that he will follow an existing connecting road, but if theroad is blocked, he will at least try to find a road around the blockedroad. The blocked condition may already have been identified in aprevious measurement by another road user, or by any other means.

If only measuring short stretches of a route segments between two pointsA and B located far away from each other, there will be many shortstretches between a pluralities of junctions located along the route. Itis an intuitive derivable fact that if a short stretch of route, forexample between junction A towards junction B, have slow moving trafficthere is a high probability that there are slow moving traffic out ontoroads from junction B as well. If one also takes into account effects ofcity planning of the road network there is even a higher probability ofslow moving traffic on the roads out of junction B if the junction ispart of such a planned route.

For example, if there are 10 road segments between A and B this will pr.definition imply that there are nine junctions between A and B. It isnot necessary to measure traffic flow conditions on all interconnectingroad segments since the traffic flow condition in one end of the routebetween A and B is probably identical to the traffic flow conditions inthe other end of the route. However, qualifying such a situation wouldmaybe require a measurement of the first road segment out of junction Aon the route to B, and a measurement of the last road segment connecteddirectly to junction B. However, if one knows that this route is along aplanned traffic route in the city, it is not necessary to measure morethan one segment somewhere on the planned traffic route since most ofthe traffic is on the main roads. Side roads and sub-roads being part ofthe planned traffic routes of the city may be more random, and mayrequire measurement points in junctions connected with main roads tocatch traffic development in and out of the main planned traffic routesinstead of arranging measure point on the main roads themselves.Further, having an overview of the traffic situation in areas around themain roads makes it possible to provide live information about availablealternative routes avoiding the main roads as much as possible.

In an example of assessing the number of road users that need toparticipate in a measurement scheme according to the present invention,if a whole area of a city is treated as equal important, it wouldrequire an evenly distributed number of road users over the whole city.In for example Oslo city, the core of the city, which often have trafficproblems, is an area of 25 kilometres multiplied with 25 kilometres. Ifthis square is divided into 10 multiplied with 10 square areas eachsubarea would be 2, 5 kilometres multiplied with 2,5 kilometres. Drivingfrom one end to the other end, from any one end to anyone of the otherends would imply driving for about three to four minutes. The speedtypically varies between 30 km/h to 50 km/h. Distributing one hundredroad users in respective sub squares can then provide qualified trafficflow measurements of at least one road segment in the respective subsquares every three minutes in average. This number should be comparedwith the fact that there is about 500 000 people living in Oslo City anda large part of the population have cars, even two cars in largerfamilies.

If one takes into account the main road aspect of cities, it is possibleto allocate fixed measuring areas based on typical traffic patternssurrounding the main roads of the city. The need of providing qualifiedmeasurements in specific allocated areas can be due to a report ofincreasing traffic problems on parts of a main road, or there is areport of an accident in an area adjacent to a main road. Measurementscan also be initiated according to historical knowledge about at whichhour any rush hour actually starts.

However, the purpose when a server is instructing a road user to drivefrom a junction A to a junction B, the task is actually to “draw” a newroad between A and B if necessary due to congestions for example, andnot to verify traffic conditions as such on known existing roads betweenA and B.

However, the direction of the road out of junction A to junction B isactually unknown at the start of the process according to aspects of thepresent invention. With reference to the discussion above, it is to“draw” a path between A an B, wherein B is located closest to A amongall other junctions surrounding A. It is also important to understandthat when a road is identified and measured, the same road can“disappear” later on due to traffic developments. Therefore, all roadsbeing measured may need to be periodically measured again and again.

FIG. 2 illustrates an example of embodiment of the present invention.When a server is instructing a road user to start driving out ofjunction 14 (being junction A selected by the server), the server shouldidentify a junction B being located closest to junction A. Identifying Bis done by establishing a pluralities of sectors with centre in thestarting junction 23. Then the sever starts searching in every sectorfor a candidate junction being a junction located geographically closestto the starting junction among the candidate junctions inside therespective junctions. In FIG. 3 junction 24 in sector 20, 25 in sector21, junction 26 in sector 22, junction are all candidate junctions forbeing junction B. In FIG. 3 junction 24 is the junction fulfilling thedistance requirement.

As can be seen in FIGS. 3 and 4 sectors are overlaid another junctionand for example, another road user is instructed to drive to anotheridentified junction B as discussed with reference to FIG. 2. Whenever aroad user reaches a destination B according to the present invention,the road user qualifies the road traffic measurements obtained by themovements of a car the road user is driving from A to B, for example bysending a text message and/or answering standardized questions issued bythe server. If there is a congestion for example on a road segmentbetween A and B, the road user may identify and follow an alternativeroute around the problematic area. The server can then track GPSpositions following the alternative route, hence a “new” road betweenjunction A and B is established. Measurements are repeated, and when thecongestion is over the original road segment is probably identified asthe road between A and B by another road user participating in themeasurement scheme according to the present invention.

Therefore, in an example of embodiment of the present invention, aserver is configured to identify a distribution of junctions, which areselected in a limited area of for example a city. The distribution canbe based on a rule of even distribution making all junctions equallyimportant in the area, or the distribution of junctions are reflectingconnectivity to main roads in the area. Any other rule of distributingor selecting junctions are within the scope of the present invention.

In each junction, being part of the selected distribution a sector withcentre in the junction is established. The number of sectors can bevaried according to the complexity of the road topology around ajunction. If there are many, short roads interconnecting many junctionsthe number of sectors can be low. If there are fewer roads and junctionsin an area the number of sectors can be lower. Then the serveridentifies the closest junction B relative to the staring junction A.

Then road users being identified or is enlisted to be part of themeasurement scheme according to the present invention can be tracked bythe server to be close to a starting junction A, or be instructed by theserver to drive to a certain starting junction A. The server is thentracking GPS data sent from the road user and when the road user reachesa destination B the measurements are qualified as discussed above. Sinceroads may be blocked, recorded movements of the road user reflectspossible alternative roads with acceptable transport capacity. Keepingtrack of routes and the measurements followed by respective road userscan be done in a directed graph as depicted in FIG. 5. Each node,representing junctions, of the graph may store measurement values ofrespective traffic flow parameters and the road user qualifiers togetherwith other information influencing traffic flow. For example, temporaryroadwork observed by a road user during measurements, or that a road ischanged to a one-way traffic road etc.

It is possible to acquire traffic flow measurements from a plurality ofroad users and store results in the respective nodes of the graph. Thenstatistical analysis can be applied to provide traffic flow predictionsfor example.

Then by parsing the recorded traffic measurements on a regular basis, ordue to an external request to a server, the server can provide updatesof traffic flow conditions. The result of the assessments can be used toupdate maps of the areas by for example colouring blocked roads withred, open roads with green and roads in transitional sate with yellow,as known in prior art. It is also possible to compare the directed graphas depicted in FIG. 5, and update visibility of roads by for example,removing temporally a blocked road from the map, and also identify roadsthat has not been measured. Instead, it can be possible to display anindication based on historical knowledge of traffic situations onnon-measured roads, for example if it is known that a road usually havelow traffic load, the road can be displayed with green dots along theroad segment.

It is within the scope of the present invention to distribute trafficflow assessments to users of the method and system thereof in any formrequested by the users of the system.

Another interesting aspect of the method of establishing sectors aroundjunctions and searching for road connections in respective sectors iswhen making a map out of information of location of junctions in anarea. A map-generating algorithm would then start by identifying a setof points, which can be junctions and their geographical location, forexample the GPS position of junctions or points. Then the method ofapplying sectors in respective junctions or points are applied.

Therefore, the following method steps can be used when making a map:

-   -   specify a region;    -   identify sample points (for example junctions) that cover the        region;    -   create X equivalent sectors around each of the sample points        that cover the full circle;    -   for each sample point A and each of its sectors: identify k        closest points in a sector and request shortest paths from/to A        to the k closest points;    -   store all paths into a directed graph, wherein the nodes        represent the points;    -   verify stored graphs by tracking cars following routes between        nodes of the stored graph.

The only major difference between the method of drawing a map asdisclosed above and providing traffic flow measurements is the laststep. According to a method of measuring traffic flow conditionsaccording to the present invention, the method comprise the followingmethod steps:

-   -   specify a region;    -   identify sample points (for example junctions) that cover the        region;    -   create X equivalent sectors around each of the sample points        that cover the full circle;    -   for each sample point A and each of its sectors: identify k        closest points in a sector and identify shortest paths from/to A        to the k closest points;    -   store all paths into a directed graph, wherein the nodes        represent the points;    -   update traffic flow measurements and road user qualifiers in        nodes of the graphs by tracking cars following respective        shortest routes between shortest path between a node A and a        closest point k in each respective sector of selected sampling        points in the region.

Although the present invention has been described in connection with thespecified embodiments, it should not be construed as being in any waylimited to the presented examples. The scope of the present invention isset out by the accompanying claim set. In the context of the claims, theterms “comprising” or “comprises” do not exclude other possible elementsor steps. In addition, the mentioning of references such as “a” or “an”etc. should not be construed as excluding a plurality. The use ofreference signs in the claims with respect to elements indicated in thefigures shall also not be construed as limiting the scope of theinvention. Furthermore, individual features mentioned in differentclaims, may possibly be advantageously combined, and the mentioning ofthese features in different claims does not exclude that a combinationof features is not possible and advantageous.

1. A computer implemented method providing measurements of traffic flowconditions on roads selected by linear road sampling, wherein a servercomprising the computer implemented method is configured to executesteps of: a) specify a region covering a road system to be measured; b)identify sample junctions that cover the region according to a specificrule of coverage; c) create a plurality of equivalent sectors aroundeach of the sample junctions that cover the full circle around therespective sample junctions; d) for each sample junction and each of itssectors: identify closest junctions in a sector and identify shortestpaths from/to sample junction to the closest junctions; e) store allpaths into a directed graph, wherein the nodes represent the junctions;f) update traffic flow measurements and measurements qualified by roadusers in nodes of the graphs by tracking cars following respectiveshortest paths identified in step d).
 2. The method according to claim1, wherein step f) further comprises identifying an alternative pathbetween the sample point and the closest junction if a measured orotherwise identified traffic flow condition indicates that the pathrecorded in the direct graph is blocked.
 3. The method according toclaim 1, wherein the step of tracking cars comprises a step wherein theserver is configured to select a specific road user to be part of themeasurement.
 4. The method according to claim 1, wherein the rule ofstep b) comprises selecting sample points being in sub regions adjacentto main roads in the region.
 5. The method according to claim 1, whereinthe rule of step b) comprises selecting sample points being evenlygeographically distributed in the selected region.
 6. The methodaccording to claim 1, wherein step f) is executed periodically.
 7. Themethod according to claim 6 wherein step f) is executed according to anexternal request to the server.
 8. The method according to claim 1,wherein step f) verify stored graphs by tracking cars following routesbetween nodes of the stored graph, thereby enabling drawing of a map. 9.A server system configured to execute a method according to claim
 1. 10.A server system configured to execute a method according to claim 8.